Remove Attachment Scope
Fixed some minor bugs
Scope will be implemented later in v3. It doesn't make sense to have
the scope handling being per attachment. Different actors can post
the same attachment with different scopes. The attachment controller
will assume the highest level of scope applied to the attachment and
the rest will be handled at the note level.
Motivation:
* Remove title from attachment, as it's part of the relation between attachment and note.
* Remove actor from attachment, many actors may publish the same attachment.
* Remove is_local from attachment, as it's part of the relation between attachment and note.
* Remove remote_url from attachment, different urls can return the same attachment.
Addition:
* Attachment now has a lives attribute, it's a reference counter with a nicer name
* GSActorToAttachment
* GSActorToRemoteURL
* RemoteURL
* RemoteURLToNote
* RemoteURLToAttachment
* AttachmentToNote now has a title attribute
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